1. Field of the Invention
The present invention relates to a pneumatic tire, and more particularly to a pneumatic tire which can maintain low rolling resistance and is improved to prevent a separation at an end of an carcass ply (ply-end separation) throughout the entire term of tire use, a pneumatic tire which is remarkably improved to prevent the fracture of the hard bead filler in the pneumatic tire caused by tire use and shows improved controllability and durability in the last stage of tire use, and a pneumatic tire which comprises a belt layer having excellent durability.
2. Description of Related Art
With automobile functions and performance becoming continuously enhanced, tires are strongly required to provide a decrease in weight and in rolling resistance (decrease in fuel consumption), and an increase in durability.
For decreasing the weight and cost of a tire, and for improving riding quality, a low turnup structure is known in which an end of a carcass ply (ply-end) is disposed at a height of 10 to 40 mm. A tire having this structure, however, with high strain at a ply-end part involves the possibility of ply-end separation causing problems with the tire. As a method of preventing this problem, a method in which sidewall compound is adhered beneath a gum chafer to relax strain, i.e., the method of so called "reversed side structure", has been used.
As a method of preventing the above problem by adjusting the formulation of the rubber composition coating carcass ply cords (ply coating rubber), improvements in resistance to crack growth and in resistance to aging of the rubber composition itself have been examined. The use of higher grade carbon black has been examined for improving the resistance to crack growth, and a method of effective vulcanization (EV cure) has been examined for improving resistance to aging.
In the reversed side structure, another piece of side rubber composition, which is called as "a cover strip" should be placed in order to prevent the ozone cracking of the gum chafer around the boundary area of the sidewall compound and the gum chager, as illustrated in FIG. 3.
The addition of this extra component causes an extra step and decreases productivity. The method of using higher grade carbon black to improve properties of a rubber composition tends to increase the rolling resistance because of the increase in heat buildup of the rubber composition. This method also adversely affects the workability in calendering in tire production. Further, the method of improving the resistance to breakage by the EV cure decreases the resistance to crack growth.
Various formulations have been examined for various parts of a tire. For the vulcanization accelerator, Japanese Patent Application Laid-Open (JP-A) No. 58-87138 discloses that the resistance to thermoset and the appearance of a tire in the last stage of tire use can be improved by using a rubber composition comprising a specific vulcanization accelerator and a specific antioxidant in a tire tread. In the above specification, nothing is disclosed about ply coating rubber, however, and neither the rolling resistance nor durability of a tire are examined. Japanese Patent Application Laid-Open (JP-A) No. 56-139542 discloses that the scorching of a rubber composition during mixing can be prevented by using a rubber composition comprising a specific vulcanization accelerator. In this specification, however, no description or suggestion is found on ply coating rubber or properties of a tire.
On the other hand, a pneumatic tire is required to have improved controllability in addition to improved durability.
For example, a tire, such as radial tire, in which the the carcass is composed of a small number of plies, generally has a structure such that a bead filler 24 which is made of hard rubber and has a damping property is disposed on a bead wire 16, and a carcass ply 14 is disposed along the inside of the bead filler 24 and is turned up at the bottom of the bead wire 16 to integrate the entire bead part as shown in FIG. 1. When a tire has this structure, stress tends to concentrate excessively to the turnup part 20 because of the inner pressure of the tire, the force applied in every rotation of the tire, and the irregular deformation by external force applied in the direction parallel to the road surface, and the concentrated stress accounts for the ply-end separation during tire use. To overcome this problem, a structure is proposed, for example, in Japanese Patent Application Publication (JP-B) No. 57-30681, in which the turnup part of the carcass ply is disposed only at a periphery of the bead wire, and the carcass and the bead wire are made to adhere tightly, and a bead filler made of a hard rubber (rubber stock) is disposed along the outside of the combined structure to integrate the bead part.
It is well known that fatigue which causes fracture is decreased and controllability is improved when an ultrahard rubber is disposed in the bead part. A hard rubber, however, generally has a smaller elongation at the time of the cutting (Eb) than a soft rubber. Moreover, the resistance to fracture of a hard bead filler is further decreased as tire is used longer. Therefore, there is the possibility that the durability of a bead filler may be decreased in the last stage of tire use.
For suppressing the decrease in resistance to fracture of a hard bead filler due to driving, an effective vulcanization system (EV cure system) can be used. In this method, a larger amount of a vulcanization accelerator is used in a rubber composition to decrease the amount of sulfur. Therefore, the amount of free sulfur which does not take part in cross-linking is decreased. As a result, hardening caused by additional cross-linking of a rubber composition under the tire use is prevented, and a decrease in Eb of a rubber composition is prevented. According to this method, Eb of a hard bead filler can be improved. The resistance to crack growth, which is another factor relating to the fracture of a bead filler, is decreased, however. This method therefore does not sufficiently improve the resistance to fracture of a hard bead filler in the last stage of tire use.
As another method for suppressing decrease in the resistance to fracture of a hard bead filler, there is a method in which carbon black having a smaller particle diameter is used as a filler in a rubber composition. This method is widely known as a method for raising the level of overall breaking properties of a bead filler part by increasing the reinforcing property of carbon black. Although the breaking properties of a bead filler can be improved by using this method, a problem arises in that the flow property of rubber composition becomes markedly inferior because of the highly reinforcing property of carbon black and workability is deteriorated to a great extent.
As described above, despite the various attempts made, a tire satisfying both controllability and durability, which are important properties required for a pneumatic tire, in the last stage of tire use has not been obtained.
On the other hand, in the belt layer used in a radial tire which comprises steel cord and rubber composition covering the steel cord (belt coating rubber), it is important that the adhesion between the steel cord and the belt coating rubber is maintained at a high level. It is known that a decrease in adhesion decreases the durability of the belt layer which, the property of the rubber composition at age, in turn, decreases he durability of the tire. For improving the adhesion between steel cord and the belt coating rubber, a method of increasing the amount of sulfur used in the rubber composition as a vulcanizing agent has heretofore been used. It is found, however, that this method has a problem that aging property, particularly heat aging property, of the belt coating rubber is deteriorated so that belt-edge separation caused by aging takes place in the last stage of tire use, although adhesion is improved.
To prevent the aging of a rubber composition, a technology in which the aging of tread rubber and side rubber is suppressed and appearance of a tire in the last stage of tire use is improved by using a combination of a specific antioxidant and a specific vulcanization accelerator is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 58-87138. No examination has been made, however, on the adhesion of rubber to a metal, such as steel cord.
As described above, it also has strongly been desired that a tire which has excellent durability, which is an important property required for a tire, particularly a tire which can prevent a decrease in strength in the last stage of tire use caused by the deterioration of the belt layer is developed.